R- Rose Comb
I already covered the smooth comb gene in detail here.
The rose comb gene, represented by R, is dominant over the wildtype single comb.
A rose comb will be wide in the front with a long, narrow "leader" sticking off the back. It is covered in large bumps.
The most classic example of the rose comb is found in Rosecomb Bantams. (Yes, Rosecomb Bantam is the breed name.)
With the addition of the recessive Smooth gene, the rosecomb will be smaller and the bumps will be smaller and smoother. Also, often the leader won't be so prominent.
There are a couple different alleles of Rosecomb. The original mutation affects the comb shape and also alters the sperm flagellum (making it more difficult for sperm to swim.) This reduces fertility of rose comb males and means you have to put in more effort to get fertile eggs out of your birds. This doesn't harm the birds though, it's only the sperm which are altered, not their precious genetic cargo.
However, there was a crossover mutation with wild type producing a rose comb allele that only affects comb shape, not sperm motility.
Unfortunately, you can't tell which allele a bird has by looking at it.
By the way, Cushion Combs are what you get when you combine the rose comb and pea comb genes.
R/RP/P
